The telecommunications industry is in the process of developing a new generation of flexible and affordable communications that includes high-speed access while also supporting broadband services. Many features of the third generation (3G) mobile telecommunications system have already been established, but many other features have yet to be perfected.
One of the systems within the third generation of mobile communications is the Universal Mobile Telecommunications System (UMTS) which delivers voice, data, multimedia, and wideband information to stationary as well as mobile customers. As can be seen in FIG. 1, the UMTS architecture consists of user equipment 102 (UE), the UMTS Terrestrial Radio Access Network 104 (UTRAN), and the Core Network 126 (CN). The air interface between the UTRAN and the UE is called Uu, and the interface between the UTRAN and the Core Network is called Iu.
Evolved UTRAN (EUTRAN) is meant to take 3G even farther into the future. EUTRAN is designed to improve the UMTS mobile phone standard in order to cope with various anticipated requirements. EUTRAN is frequently indicated by the term Long Term Evolution (LTE), and is also associated with terms like System Architecture Evolution (SAE).
Information about LTE can be found in 3GPP TR 25.913 (V7.2.0, December, 2005), Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN and also in 3GPP TR 25.813 (V0.1.0, November 2005), Evolved UTRA and UTRAN—Radio interface protocol aspects, both of which are incorporated herein by reference in their entirety. UTRAN and EUTRAN will now be described in some further detail.
The UTRAN consists of a set of Radio Network Subsystems 128 (RNS), each of which has geographic coverage of a number of cells 110 (C), as can be seen in FIG. 1. The interface between the subsystems is called lur. Each Radio Network Subsystem 128 (RNS) includes a Radio Network Controller 112 (RNC) and at least one Node B 114, each Node B having geographic coverage of at least one cell 110. As can be seen from FIG. 1, the interface between an RNC 112 and a Node B 114 is called Iub, and the Iub is hard-wired rather than being an air interface. For any Node B 114 there is only one RNC 112. A Node B 114 is responsible for radio transmission and reception to and from the UE 102 (Node B antennas can typically be seen atop towers or preferably at less visible locations). The RNC 112 has overall control of the logical resources of each Node B 114 within the RNS 128, and the RNC 112 is also responsible for handover decisions which entail switching a call from one cell to another or between radio channels in the same cell.
In UMTS radio networks, a UE can support multiple applications of different qualities of service running simultaneously. In the MAC layer, multiple logical channels can be multiplexed to a single transport channel. The transport channel can define how traffic from logical channels is processed and sent to the physical layer. The basic data unit exchanged between MAC and physical layer is called the Transport Block (TB). It is composed of an RLC PDU and a MAC header. During a period of time called the transmission time interval (TTI), several transport blocks and some other parameters are delivered to the physical layer.
Generally speaking, a prefix of the letter “E” in upper or lower case signifies the Long Term Evolution (LTE). The E-UTRAN consists of eNBs (E-UTRAN Node B), providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs interface to the access gateway (aGW) via the S1, and are inter-connected via the X2.
An example of the E-UTRAN architecture is illustrated in FIG. 2. This example of E-UTRAN consists of eNBs, providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are connected by means of the S1 interface to the EPC (evolved packet core), more specifically to the Mobility Management Entity (MME). The S1 interface supports a many-to-many relation between MMEs and eNBs. The MME in the example of FIG. 2 is one option for the access gateway (aGW).
In this example of E-UTRAN, there exists an X2 interface between the eNBs that need to communicate with each other. The eNB may host functions such as radio resource management (radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to UEs in both uplink and downlink), selection of a mobility management entity (MME) at UE attachment, scheduling and transmission of paging messages (originated from the MME), scheduling and transmission of broadcast information (originated from the MME or O&M), and measurement and measurement reporting configuration for mobility and scheduling. The MME may host functions such as the following: distribution of paging messages to the eNBs, security control, IP header compression and encryption of user data streams; termination of U-plane packets for paging reasons; switching of U-plane for support of UE mobility, idle state mobility control, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of NAS signaling.
According to recent developments in this field, the user equipment (UE) can measure any reference signal (RS) from a first antenna (#1) and a second antenna (#2), with the exact pilot/frame structure given in 3GPP TS 36.211 V. 1.0.0 (2007 March) Physical Channels and Modulation (Release 8) which is incorporated herein by reference in its entirety. Orthogonal Frequency Division Multiplexing (OFDM) symbols bearing this reference signal (RS) occur 4 times per unicast sub-frame and only once in Multicast Broadcast Single Frequency Network (MBSFN) sub-frames. The accuracy and reliability of mobility measurements depends on the number of RS resource elements eligible for measurements, and on the UE awareness of presence or absence of some RS resource elements (e.g. in MBSFN sub-frames). These mobility measurements have to be carried out on RS resource elements of the cell on which the UE is camping on, as well as on corresponding neighbor cells.
Various solutions to this problem have been attempted. For example, one technique is to signal full MBSFN sub-frame allocation of a cell on this cell's primary Broadcast Channel (P-BCH). However, that technique is very costly from the overhead point of view, because the P-BCH is a very robust channel and each P-BCH bit consumes a non-negligible part of a cell's capacity. Furthermore, for measuring RS from neighbor cells, this requires that the P-BCH from neighbor cells be received before and during measurements which might be unreliable and adds extra complexity.
A second technique is to measure the RS only in the first OFDM symbol of each sub-frame. However, according to that technique, the number of RS elements eligible for measurements is always reduced by a factor of 4, which will incur measurement inaccuracy.
A third technique is to signal a per carrier MBSFN/non-MBSFN indication. Therefore, in the MBSFN case, only the first RS OFDM symbol of each sub-frame would be used (see second technique described above), and in the non-MBSFN case up to 4 RS OFDM symbols could be used. This means that in the latter case all available and eligible RS elements can be measured in unicast carriers. However, according to this third technique, the number of RS elements eligible for measurements is reduced by a factor of 4 in any mixed MBSFN/unicast carrier (including carriers where the MBSFN area includes only a couple of cells or including carrier-wide MBSFN area(s)).